Turbo Guide: S300 vs S400, Sizing & Fitment

A turbo is the single biggest lever on how a diesel truck breathes, tows and responds. It is also one of the easiest parts to order wrong. A turbo charger that looks right on paper can still fail to bolt up, spool late, or run hot if the frame, turbine housing, flange, drain or discharge doesn't match the truck. This guide covers how turbochargers work, how the main types differ, what frame names like S300 and S400 mean, how sizing is approached, and what has to match before you order.

Black Sheep Industries machines turbo hardware in Canada under the True North Turbos name, and many of our customers are American. That's why measurements here are metric first with US standard in parentheses, and why power goals are discussed in crank horsepower. Use this page as your starting point, then move on to the S300 frame guide, the S400 frame guide, or the S300 vs S400 comparison guide for frame-level detail.

What Is a Turbocharger and What Does It Do?

A turbocharger is an exhaust-driven air pump. Hot exhaust gas leaves the engine and spins the turbine wheel. A shaft running through the centre housing connects that turbine to the compressor wheel, which draws in fresh air and compresses it. That denser air passes through the intercooler and into the engine, where more air allows more fuel to be burned efficiently. A diesel turbo doesn't add power by itself. It supplies the air that lets the engine make more of it.

The centre section spins at very high speed, so it relies on engine oil for lubrication and cooling. Some centre housings also have coolant ports, but coolant provision has to be confirmed for each cartridge and application rather than assumed. Turbochargers are simple in concept and unforgiving in the details, and those details are what the rest of this guide covers.

Boost Is Not the Same as Airflow

Boost is a pressure reading. Airflow is how much air mass the engine actually receives. Two trucks can show the same gauge boost and still be moving different amounts of air, because air temperature, engine speed, intercooling and the efficiency of the whole system all change what that pressure means. Hot, inefficiently compressed air carries less oxygen than cooler, efficiently compressed air at the same boost number.

That's why you shouldn't choose a turbo by boost target alone, or by a dyno screenshot from someone else's truck. We plan around crank horsepower, not wheel horsepower. Wheel figures depend on the dyno, the correction method and the drivetrain, so they make a poor basis for choosing a turbo.

Wastegates and Variable-Geometry Turbos

A fixed-geometry diesel turbo normally uses a wastegate, either internal or external. The wastegate lets some exhaust bypass the turbine, which limits shaft speed and boost. A variable-geometry turbo, called VGT or VNT depending on the manufacturer, works differently. It changes the position of vanes around the turbine to control exhaust flow, instead of, or in addition to, using a wastegate.

Many Cummins, Power Stroke and Duramax trucks of the relevant eras left the factory with a VGT. Replacing a factory VGT with a fixed-geometry turbo changes spool behaviour, exhaust braking and what the engine control unit expects to see. It's a system change, not a cartridge replacement, and it needs to be planned that way. This page doesn't offer tuning advice or guidance on removing emissions equipment.

Main Types of Turbochargers for Diesel Trucks

If you're shopping for a truck turbo, you'll run into several configurations: fixed geometry and VGT/VNT, journal bearing and ball bearing, and single, twin, sequential and compound arrangements. This section keeps things high level. Detailed frame decisions live in the S300 and S400 guides.

Fixed Geometry vs VGT/VNT Turbochargers

A fixed-geometry turbo has a turbine housing with a set flow area. Its boost is controlled by a wastegate that bleeds off exhaust energy when needed. A VGT or VNT turbo has movable vanes that open and close, tightening the flow path at low engine speed for quick response and opening it up as exhaust flow rises. A typical aftermarket S300 or S400 is fixed geometry with an internal or external wastegate.

Whether a fixed-geometry conversion is the right move depends on the truck, not on a general rule. A factory VGT replacement decision involves fitment, exhaust braking, tuning expectations, pedestal and manifold compatibility, and the supporting hardware around the turbo. Treat a conversion as application-specific rather than a universal upgrade.

Fitment warning: If you don't know your factory turbo configuration, manifold flange, pedestal, downpipe or drain setup, stop before ordering and send those details through the Turbo Spec Form. Guessing is how mismatched flanges and unusable downpipes happen.

Journal Bearing vs Ball Bearing Turbos

Journal-bearing turbos are the workhorses of diesel applications. They're generally tolerant and durable, but they need a steady supply of oil flow and tend to be slower to light than ball-bearing designs. Ball-bearing cartridges can improve response and use less oil, but they aren't automatically the right choice for every diesel, particularly in high-exhaust-temperature applications.

The TNT3R runs a journal bearing with a 360-degree thrust bearing. Always confirm current cartridge specifications for any product you're considering, since bearing design is a product-level detail that should come from the current spec sheet, not from a generic comparison.

Single, Twin, Sequential and Compound Turbo Systems

A single turbo is the standard configuration for street and towing trucks, and it's where most buyers should start thinking. Parallel twins split the exhaust flow between two turbos and are usually found on V-engine layouts. Sequential systems stage a small and a large turbo, valving between them as engine demand changes. They're less common in this aftermarket context.

Compound systems place two turbos in series. The turbo mounted on the manifold is the high-pressure (HP) charger, and it feeds already-compressed air onward. The turbo open to atmosphere is the low-pressure (LP) charger, which does the first stage of compression. On Black Sheep frames, the usual architecture is the S300/TNT3R as the HP charger and the S400/TNT4R as the atmospheric charger.

Turbo Frame Sizes Explained: S300, S400 and TNT Frames

The frame number tells you which turbo family you're dealing with. It doesn't describe the complete wheel set, turbine, housing or fitment package. Two turbochargers on the same frame can differ in compressor, turbine, housing and flange, and the naming convention is where most ordering errors start.

In Black Sheep's naming, the TNT3R is the 300 frame, in the S300 class. The TNT4R is the 400 frame, in the S400 class. Keeping those equivalents straight prevents the most common interchangeability mistakes. Knowing the frame is the first step. Everything after it still has to be matched.

Small Frame, S300 / TNT3R and S400 / TNT4R

Small-frame turbos sit below the S300 class. One identifying measurement is an oil-drain bolt centre of 38 mm (1.50 in).

The S300 / TNT3R is the mid-frame option, and it is T4 turbine inlet only. The S400 / TNT4R is the large-frame option. It's available with a T4 or T6 inlet, depending on the turbine housing.

Don't choose a frame on the assumption that bigger is better. The right frame depends on your real RPM range, how much you tow, how the truck is driven on the street, what top-end flow you need, your drivetrain, and whether the surrounding hardware is compatible.

What S366, S376 and S476 Actually Mean

In the S-series convention used on these frames, the first number tells you the frame (300 or 400), and the two digits after it give the compressor inducer diameter in millimetres. An S366 is a 300-frame turbo with a 66 mm compressor inducer. It is not a "366 frame."

That shorthand leaves out a lot. It doesn't tell you the compressor exducer, turbine wheel, housing A/R, discharge size, oil-drain arrangement or fitment. A common misconception involves the S376 and the S476. Both can share a 76 mm compressor inducer, but they're different frames with different turbine, housing, discharge and oil-drain considerations. They are not interchangeable turbos with a different label. For more on that distinction, see the S300 vs S400 comparison guide.

S300 vs S400: Which Turbo Frame Is Right for Your Build?

This is a directional overview, not a full head-to-head. The choice between S300 and S400 should come from how the truck is used, its RPM band, the manifold flange, discharge, oil drain, intercooler capacity and drivetrain support. We're deliberately not publishing universal horsepower ranges or recommending individual wheel combinations here, because those depend on the full system.

For the full comparison, use the S300 vs S400 comparison guide. It covers the frame-to-frame decision in depth so this page doesn't have to.

When an S300 / TNT3R Frame Usually Makes Sense

An S300 / TNT3R tends to suit towing, street response and midrange drivability, where earlier spool matters. It requires a T4 turbine inlet, so confirm that your manifold flange matches before going further. A smaller frame isn't automatically restrictive. When it matches the truck's actual operating range, it can be the better-suited choice. If you want S300 wheel, housing and map details, the S300 frame guide covers them.

When an S400 / TNT4R Frame Usually Makes Sense

An S400 / TNT4R tends to suit larger airflow needs, higher-RPM use, strip applications, competition and the atmospheric position in a compound setup. Its housings may use a T4 or T6 turbine inlet depending on the housing, so confirm which one you need. Later spool can be perfectly acceptable when gearing, converter or clutch, and intended RPM range keep the engine where the turbo works best. For S400 wheel, housing and map details, the S400 frame guide goes deeper.

How to Size a Turbocharger for a Diesel Engine

If you're asking how to choose a turbo, the core principle is this: match the turbo to the engine's airflow demand across the RPM range where the truck actually works. Turbo sizing is not about a peak number. Avoid sizing from a single dyno result, a boost figure, a social-media recommendation or a wheel-horsepower screenshot. When we talk about build goals, we use crank horsepower consistently.

Compressor Sizing Basics

A compressor map plots pressure ratio against mass flow. The surge line sits on the left, where flow is too low for the pressure being asked of the compressor. Choke sits on the right, where the compressor can't flow any more air. Between them are efficiency islands, and the goal is for the engine's operating line to stay in a useful part of the map from spool through peak RPM.

Compressor inducer is the headline sizing figure, but the exducer shapes the map and is easy to overlook. No single airflow-to-horsepower conversion factor is safe to publish as a universal diesel rule, because the real relationship varies with air-fuel ratio, intercooler effectiveness and altitude. For map-specific analysis, go to the S300 compressor and housing guide or the S400 compressor and housing guide, not to a rule of thumb.

Turbine Wheel and A/R Basics

The turbine wheel and the turbine housing A/R influence spool response, exhaust flow, drive pressure and top-end capability. The general trade-off is straightforward. A smaller A/R spools sooner but can raise drive pressure. A larger A/R responds later but generally supports more top-end flow.

On a diesel, drive pressure, exhaust-gas temperature and intercooler capacity are key considerations, especially when towing. Drive pressure running well above boost is a limit to respect. A larger frame is not automatically a better towing or street choice, and often it's the opposite.

Choosing for Towing, Street, Strip and Competition

Towing. Prioritize usable RPM range, response, exhaust-gas-temperature management, drain routing and practical fitment. A lazy, oversized single is one of the most common tow-truck mistakes.

Street. Balance spool, midrange, sound, manifold compatibility and day-to-day drivability. Noise and fitment matter as much as peak flow.

Strip. Later spool may be acceptable where the drivetrain and gearing keep the engine in the usable part of the map. Discharge size and intercooler capacity can become the limits.

Competition and compounds. The normal Black Sheep architecture is an S300/TNT3R high-pressure turbo over an S400/TNT4R atmospheric turbo. The critical rule is that the high-pressure turbo should be sized smaller than the single-turbo choice you'd make for the same peak crank-horsepower goal. Both turbos see related mass flow, but the pressure ratio is split between them, and the HP unit is fed air that's already compressed. Sizing the HP charger like a single typically gives late response and pushes drive pressure and exhaust-gas temperature the wrong way. Specific pairings belong in the S300 and S400 frame guides, not here.

Truck Turbo Fitment: What Must Match Before You Order

Hardware compatibility matters as much as compressor and turbine sizing. A truck turbo that's well matched to the engine can still fail to install, or fail in service, if the flange, drain, discharge, oil feed or intercooler requirements are ignored. Every turbo charger decision should be checked against the hardware around it before it's ordered.

Frame	Turbine inlet	Oil-drain bolt centre	Discharge
S300 / TNT3R	T4 only	50 mm (1.97 in)	4.25 in marmon
S400 / TNT4R	T4 or T6, by housing	50.8 mm (2.00 in)	T4: 4.62 in (117 mm) half marmon; T6: 5.75 in (146 mm) full marmon

Turbine Inlet Flanges: T4 vs T6

"Bolt pattern" is the correct term for turbine inlet flange formats such as T3, T4 and T6. It applies to the flanges, gaskets and turbine housings themselves. S300 / TNT3R is T4 only. S400 / TNT4R can be T4 or T6 depending on the turbine housing.

Confirm your manifold flange before ordering. Don't assume. An S300 cannot be ordered as a T6.

Oil Drain Bolt Centre Measurements

Terminology matters here. The correct term is oil-drain bolt centre. It is not "oil-drain bolt pattern," and it is not "bolt circle." The measurements are:

• Small frame: 38 mm (1.50 in)
• Mid frame / S300: 50 mm (1.97 in)
• Large frame / S400: 50.8 mm (2.00 in)

The S300 and S400 drain bolt centres differ by only 0.8 mm, so you need calipers to tell them apart. Eyeballing it won't work.

Bolt centre is also separate from drain length and port size. Drain lengths come in 35 mm, 65 mm or 100 mm. Port sizes are 10AN or 12AN, with 12AN noted for the S400. An uphill drain route or an undersized drain can damage a centre housing regardless of how well the turbo is sized.

Turbine Discharge and Marmon Connections

The discharge dimensions are:

• S300: 4.25 in marmon
• S400 T4: 4.62 in (117 mm) half marmon
• S400 T6: 5.75 in (146 mm) full marmon

The 4.25 in and 4.62 in connections do not clamp together. That mismatch is a common 300-frame ordering error, usually made when a buyer assumes a downpipe built for one will fit the other. Before purchasing, check your downpipe, discharge hardware and intercooler plumbing against the turbo you're considering.

When to Stop and Confirm Fitment

Pause and get fitment confirmed if any of the following is true: you don't know your manifold flange, your oil-drain measurement, your pedestal, or your downpipe or discharge connection, or you're replacing a factory VGT. In those cases, use the Turbo Spec Form instead of guessing.

To make the process quick, have the following ready: your truck platform, engine, intended use, current turbo setup, manifold flange, oil-drain bolt centre and discharge style.

Turbocharger Applications by Diesel Platform

Factory VGT configuration, pedestal, manifold flange, downpipe, drains and supporting hardware all vary by platform and generation. For that reason, this section doesn't assign a universal truck turbo recommendation to any engine. Use it to identify your platform, then move to the right frame guide or the Turbo Spec Form.

Cummins Turbo Considerations

This covers 5.9 L and 6.7 L Cummins applications. A common buyer decision is between towing and street response on one hand, and larger single or compound ambitions on the other. Before selecting a frame, confirm the manifold, turbine housing, oil drain and supporting hardware.

Power Stroke Turbo Considerations

This covers 6.0 L, 6.4 L and 6.7 L Power Stroke applications. Factory VGT configuration and replacement requirements can vary significantly by generation. A fixed-geometry conversion should be treated as a complete-system decision, not a simple turbo swap.

Duramax Turbo Considerations

This covers LB7 through L5P Duramax applications. Manifold, downpipe, VGT and fitment details differ by platform. If your factory VGT has failed, or you're unsure what hardware is on the truck, get fitment support before buying.

Should You Upgrade or Rebuild Your Turbo?

Rebuild versus upgrade depends on the condition of your existing turbo, how you use the truck, the hardware around it and how much of the system you're willing to change. We can't diagnose an individual turbo failure remotely, and not every failed turbo should be upgraded, nor can every unit be rebuilt.

When a Rebuild May Be the Better Path

A rebuild is worth considering when you want to keep the same general system, the same fitment and the same driving purpose. Before assuming a rebuild is suitable, the centre housing, wheels, turbine housing and overall damage need to be assessed. Verify current cartridge and service options with Black Sheep rather than relying on generic assumptions.

When a Turbo Upgrade May Make Sense

An upgrade makes sense when your goals have changed. That might mean more towing capacity, a different street character, strip use or a compound build. A turbo upgrade may also require matching changes to the manifold, wastegate, downpipe, oil drain, intercooler, tuning strategy and drivetrain. Moving away from a factory VGT is a system-level change, not a bolt-on.

A useful way to decide is to ask what you want from the change. Do you want to retain current fitment, improve response, increase top-end flow or build a compound? Each answer points to a different path.

How Much Is a Turbocharger?

It's the question most buyers start with, and there's no honest universal answer. The cost of a turbocharger depends on the frame, the compressor and turbine combination, the bearing and cartridge design, the turbine housing, wastegate requirements, platform hardware, drain and feed needs, and whether you're building a single or a compound. A price quoted without those details isn't very useful.

Prices on the Canadian storefront are shown in CAD. Cross-border buyers should confirm shipping, duties and fitment requirements before ordering. For a realistic estimate of a complete configuration, browse the current Black Sheep turbo collections or request fitment help.

Common Turbo Ordering Mistakes to Avoid

Most expensive turbo mistakes come down to measurement, fitment verification and frame confusion, and nearly all can be avoided.

Sizing and Application Mistakes

• Choosing an oversized single turbo for towing because bigger must be better.
• Sizing a compound high-pressure turbo like a single, which causes late response and unfavourable drive-pressure and exhaust-gas-temperature behaviour.
• Selecting by wheel-horsepower screenshots instead of crank-horsepower goals, actual RPM range and supporting hardware.
• Ignoring drive pressure, exhaust-gas temperature, intercooler capability, gearing, converter or clutch, or intended use.

Fitment and Hardware Mistakes

• Ordering an S300 as T6, when S300/TNT3R is T4 only.
• Treating an S376 and S476 as the same turbo because both may use a 76 mm compressor inducer.
• Trying to clamp a 4.25 in S300 discharge to a 4.62 in S400 T4 connection.
• Assuming S300 and S400 oil-drain flanges interchange without measuring the 0.8 mm bolt-centre difference.
• Calling an oil-drain measurement a "bolt pattern" instead of measuring bolt centre.

Emissions, Road Use and Compliance Considerations

This section is factual and non-prescriptive. It is not legal advice, and it doesn't cover how to remove or defeat emissions equipment.

United States Considerations

In the United States, the Clean Air Act prohibits tampering with emissions controls on motor vehicles and prohibits selling parts known to defeat those controls. Owning a turbo isn't the issue. On-road emissions compliance depends on the full vehicle configuration, and this page can't certify whether any given turbo keeps a vehicle within its certified limits. Off-road use and sanctioned competition on a vehicle not used on public roads may be treated differently from public-road use. The EPA documents its defeat-device enforcement on epa.gov.

Canadian and Cross-Border Considerations

In Canada, on-road emissions enforcement can involve both federal standards and provincial rules. Requirements vary by province and may not line up exactly with US rules. If you're buying across the border, check the requirements where the truck is plated before you order.

Find the Right Turbocharger for Your Build

The path is simple. First, identify your intended use: tow, street, strip, competition or compounds. Second, confirm your frame and hardware requirements, including turbine inlet, oil-drain bolt centre and discharge. Third, shop the appropriate collection or submit your fitment details.

If your flange, drain, pedestal, discharge or VGT configuration is unknown, send it to the Turbo Spec Form rather than guessing. When you're ready to look at turbos for sale, browse the current Black Sheep turbocharger collections, or start with the S300 frame guide, S400 frame guide or S300 vs S400 comparison guide if you're still deciding on a frame.

Turbocharger FAQs

Whether you're comparing turbochargers for the first time or double-checking a fitment detail, these are the questions we hear most.

How do I know what size turbo I need for a diesel?

Start with the RPM range where your truck actually works, then consider your crank-horsepower goal, whether it's used for towing, street, strip or competition, your drivetrain and the supporting hardware. Don't size from a peak number or a wheel-horsepower screenshot. If you're unsure, send your details through the Turbo Spec Form.

What is the difference between an S300 and an S400 turbo?

They're different frame classes. The S300 / TNT3R is the mid-frame option and is T4 turbine inlet only. The S400 / TNT4R is the large-frame option and is available with T4 or T6 depending on the housing. Their discharge connections also differ. In general, the S300 leans toward earlier response and the S400 toward higher airflow and top-end, but the right choice depends on how the truck is used. The S300 vs S400 comparison guide covers it in full.

Is a bigger turbo worse for towing?

Often it is. A larger turbo can spool later, so the engine may spend more time outside the part of the map where the turbo works well. Drive pressure and exhaust-gas temperature can also become problems when towing. A turbo that matches the RPM range where the truck pulls usually serves a tow rig better than one sized for peak flow.

What does A/R mean on a turbocharger?

A/R describes the turbine housing's geometry, the relationship between its flow area and the radius at which that area sits. In practice, a smaller A/R generally spools sooner and raises drive pressure, while a larger A/R responds later and usually supports more top-end flow.

Should I rebuild my turbo or upgrade it?

It depends on the damage to the existing unit, whether you want to retain your current fitment, your build goals and the supporting hardware. A rebuild suits keeping the same system and purpose. An upgrade suits changed goals, though it may require other changes. Confirm the options with Black Sheep before deciding.